Thanks for the help, fellas!
I dug out the Dremmel and started trimming, going about 1/4" at time. Then drove out to a nearby parking lot to check the SWR again. Then back and forth a few more times.
I got ch40 down into the 1.6 neighborhood, but of course, that cost me on ch1 (as it also came up to around 1.6).
Ch19 checks around 1.1, so I called it good enough for now. I chose the magnet mount because it's a temporary installation anyway.
That is a perfect example of 'bandwidth'. This is an example bandwidth shown graphically, where the tuned frequency is 300MHz. You can see how the SWR increases off it's center.
View attachment 43972
Your channel 19 is 27.185MHz and the ends are 26.985MHz and 27.405MHz. Most times bandwidth is defined as the range over which 2.0:1 SWR can be achieved, but there is really no hard-fast rule. If your radio can tolerate 3.0:1 then your antenna bandwidth requirements can adjust accordingly.
So technically you have more than 420KHz of rated bandwidth using the 2:1 definition. That's not really bad at all for a compromised HF (mag mount = poor mount, physically very short, etc.) antenna. Not to mention a SWR of 2.0:1 represents about 0.5dB of loss relative to a perfect match, so at your 1.6:1 ends you are radiating about 95% of your radio's output power and getting about 99.75% at 1.1:1 at channel 19.
You in fact have a perfectly tuned CB antenna for general coverage, in that you really can't do much better without going to a physically longer antenna. I personally tuned mine for channel 9 and so my upper end is a poorer match than my lower end, but this was a calculated thing since I primarily use channel 4 and only if I have to. I could envision using channel 9 and 19 maybe, otherwise I just don't use CB. My HF radio can monitor CB and has a more sensitive receiver than any CB, so the only time I even put on a CB antenna is if I absolutely have to TX on CB, which is fairly rare anymore.
